Porter GeoConsultancy, continued its International Study Tour series of professional development courses by visiting a representative selection of the major iron deposits illustrating a range of different ore styles in southern Africa and Brazil as the first of two modules.
The tour commenced in Johannesburg, South Africa on the evening of Sunday18 April, 2010 and ended in Carajás, Pará, Brazil on the evening of Friday 30 April. Participants were able to take any 3 or more days, up to the full tour module, as suited their interests or availability.

The workshop, which was held in Johanensburg, was led by internationally renowned expert Professor Nic Beukes of the University of Johannesburg, and included coverage of the following topics:

An overview of the geological and tectonic framework of iron ore mineralisation in Southern Africa, and geological controls and distribution of significant iron deposits within the same region.

The factors influencing the development of ore grade iron mineralisation in southern Africa and the genesis of the key deposits.

Descriptions of other significant deposits not to be visited and a comparison with those that are on the itinerary.

South Africa is Africa's leading iron ore producer, with an annual output of around 33 Mt, and 'reserves' of approximately 9 billion tonnes, 45% of which are located in the Northern Cape Province. Production comes from two main areas, the Sishen mine in the Northern Cape Province, accounting for over 24 Mt of high grade 65% Fe ore per annum, and the Thabazimbi Mine in the Northern Province. Thabazimbi generally provides iron to South African domestic smelters at Vanderbijlpark and Newcastle, at a rate of around 2 Mtpa, while almost all production from Sishen is exported through the port of Saldanha north of Cape Town.

Although 700 km apart, Thabazimbi and Sishen are both developed on iron formations in the Palaeoproterozoic Transvaal Sequence, which was deposited between sometime after 2400 Ma and 2100 Ma within an intracratonic basin on the Kaapvaal Craton. The Kaapvaal Craton comprises a 3.4 to 2.8 Ga granite-greenstone basement, similar to the Zimbabwe Craton to the north which is separated by the 2.5 to 2.0 Ga Limpopo Mobile Belt. The granite-greenstone basement terrane is over lain by a series of intracratonic basins, the axes of which migrated north with time from the early "upper" Archaean to the late Palaeoproterozoic Waterberg System to the north.

The oldest of these, the Pongola System is a 10 000 m thick, approx. 3.0 Ga succession comprising a lower basaltic volcanic and lesser quartzite sequence overlain by alternating argillaceous and arenaceous sediments with intercalations of BIF. The overlapping ~9 000 m thick Witwatersrand Supergroup was deposited at between 3075 and 2715 Ma, commencing with early Dominion basaltic lavas, followed by a succession of mainly sandstones and shales with lesser locally gold bearing conglomerates. These were in turn followed by the up to 5000 m thick Ventersdorp Supergroup composed dominantly of basaltic volcanics dated at 2714 Ma.

The up to 12 000 m thick Transvaal Sequence was deposited unconformably on the Ventersdorp Sequence, and occurs in two connected depo-centres, the Transvaal and Griqualand West sub-basins, which define a 1100x350 km, NE-SW elongated area of remaining exposure. This sequence originally covered an area of approx. 500 000 sq. km.

In the Transvaal sub-basin, the Transvaal sequence commences with the up to 2000 m thick Wolkberg (or Buffalo Springs) Group of valley fill basalts and coarse clastics and lesser argillites. These are followed by the few tens to 500 m thick Black Reef Quartzite, which grades up into the around 3000 m thick Chuniespoort Group which comprises the lower up to 2000 m thick Malmani Dolomite, variably composed of dolomite and chert, and the overlying up to 600 m thick Penge Iron Formation.

The Penge Iron Formation is the host to the Thabazimbi iron deposit and is composed of alternating carbonaceous shale and macro-, meso- and micro-banded BIF (quartz- magnetite- hematite- stilpnomelane- riebeckite- minnesotaite- grunerite and ferriferous carbonates). The uppermost member of the Chuniespoort Group is represented by the dolomites, quartzites and shale of the locally preserved Duitschland Formation. The Chuniespoort Group is unconformably overlain by the 7000 m thickness of quartzites, shales and minor basalts of the Pretoria Group and the 2-3000 m of rhyolitic lavas that make up the ~2100 Ma Rooiberg Group.

In the Griqualand West sub-basin, the Transvaal Sequence is represented by the Ghaap Group, which is unconformably overlain by the Postmasburg Group. The Ghaap Group is sub-divided into the lower interbedded silici-clastics and carbonates of the Schmidtsdrif Subgroup followed by the limestones and dolomites of the Campbellrand Subgroup. These are overlain by the Asbesheuwels Subgroup which is sub-divided into the lower Kuruman Iron Formation, composed of interlayered carbonaceous shale and a chert- carbonate- stilpnomelane- magnetite- hematite- greenalite- siderite iron formation, and the upper Griquatown Iron Formation, comprising siderite-hematite and siderite-greenalite lutites. The Asbesheuwels Subgroup is host to the giant Sishen iron deposit. The Ghaap Group is unconformably overlain by the Postmasburg Group, commencing with the thin Makganyene Diamictite, the thick Ongeluk basaltic pillow lavas, followed in turn by the jasper, BIF and sedimentary manganese deposits of the Hotazel Formation, and finally the Mooidraai Dolomite. These are in turn unconfoirmably overlain by the shales and red-bed sanstones of the Olifantshoek Group.

The Transvaal Sequence is unconformably overlain to the north by the extensive thick arkosic arenites of the 2000 to 1700 Ma Waterberg Group.

The Sishen iron ore deposit is situated in the Northern Cape Province of South Africa, 280 km north of Kimberley and 800 km north-east of its export port at Saldanha Bay north of Cape Town, to which it is connected by a 1 m gauge rail line. It is just to the SW of the township of Kathu (#Location: 27° 45' 32"S, 23° 0' 0"E).

The mine is owned and operated by Kumba Resources Limited and produces around 27 Mt pa of 65% Fe hard hematite ore with a high lump proportion, and impurity levels of generally less than 0.05% P, 2.5% SiO2 and 1.2% Al2O3.

Regional Setting

The major iron deposits of South Africa, which include Thabazimbi and Sishen that are 700 km apart, are developed on iron formations in the Palaeoproterozoic Transvaal Sequence, which was deposited between ~2.4 and 2.1 Ga within an intracratonic basin on the Kaapvaal Craton. The Kaapvaal Craton comprises a 3.4 to 2.8 Ga granite-greenstone basement, which is separated from the Zimbabwe Craton to the north by the 2.5 to 2.0 Ga Limpopo Mobile Belt. The granite-greenstone basement terrane is overlain by a series of intracratonic basins, the axes of which migrated northward with time from the early Neoarchaean to the late Palaeoproterozoic Waterberg System to the north.

The oldest of these, the Pongola System is a 10 000 m thick, ~3.0 Ga succession comprising a lower basaltic volcanic and lesser quartzite sequence overlain by alternating argillaceous and arenaceous sedimentary rocks with intercalations of BIF. The overlapping ~9000 m thick Witwatersrand Supergroup was deposited at between 3075 and 2715 Ma, commencing with early Dominion basaltic lavas, followed by a succession of mainly sandstones and shales with lesser locally gold bearing conglomerates.These were in turn followed by the up to 5000 m thick Ventersdorp Supergroup composed dominantly of basaltic volcanic rocks dated at 2714 Ma.

The up to 12 000 m thick Transvaal Sequence was deposited unconformably on the Ventersdorp Sequence, and occurs in two connected depo-centres, the Transvaal and Griqualand West sub-basins, which define a 1100 x 350 km, NE-SW elongated area of remaining exposure. This sequence originally covered an area of ~500 000 sq. km.

In the Transvaal sub-basin, the Transvaal sequence commences with the up to 2000 m thick Wolkberg (or Buffalo Springs) Group of valley fill basalts and coarse clastics and lesser argillites. These are followed by the few tens to 500 m thick Black Reef Quartzite, which grades up into the around 3000 m thick Chuniespoort Group that comprises the lower up to 2000 m thick Malmani Dolomite, variably composed of dolomite and chert, and the overlying up to 600 m thick Penge Iron Formation. The Penge Iron Formation is the host to the Thabazimbi iron deposit and is composed of alternating carbonaceous shale and macro-, meso- and micro-banded BIF (quartz- magnetite- hematite- stilpnomelane- riebeckite- minnesotaite- grunerite and ferriferous carbonates). The uppermost member of the Chuniespoort Group is represented by the dolomites, quartzites and shale of the locally preserved Duitschland Formation. The Chuniespoort Group is unconformably overlain by the 7000 m thickness of quartzites, shales and minor basalts of the Pretoria Group and the 2 to 3000 m of rhyolitic lavas that make up the ~2100 Ma Rooiberg Group, marking the stage of emplacement of the Bushveld Complex.

In the Griqualand West sub-basin, the Transvaal Sequence is represented by the Ghaap Group, which is unconformably overlain by the Postmasburg Group. The Ghaap Group is sub-divided into the lower interbedded silici-clastics and carbonates of the Schmidtsdrif Subgroup followed by the limestones and dolomites of the Campbellrand Subgroup. These are overlain by the Asbesheuwels Subgroup which is sub-divided into the lower Kuruman Iron Formation, composed of interlayered carbonaceous shale and a chert-carbonate-stilpnomelane-magnetite-hematite-greenalite-siderite iron formation, and the upper Griquatown Iron Formation, comprising siderite-hematite and siderite-greenalite lutites. The Asbesheuwels Subgroup is host to the giant Sishen iron deposit. The Ghaap Group is unconformably overlain by the Postmasburg Group, commencing with the thin Makganyene Diamictite, the thick Ongeluk basaltic pillow lavas, followed in turn by the jasper, BIF and sedimentary manganese deposits of the Hotazel Formation, and finally the Mooidraai Dolomite. These are in turn unconformably overlain by the shales and red-bed sanstones of the Olifantshoek Group.

The Transvaal Sequence is unconformably overlain to the north by the extensive thick arkosic arenites of the 2000 to 1700 Ma Waterberg Group.

Deposit Geology

The Sishen deposits are hosted by the Palaeoproterozoic Transvaal Supergroup within the Griqualand West sub-basin. It is situated on the Maremane Dome which is defined by the carbonate sequence of the Transvaal Supergroup, the Campbellrand Subgroup and the overlying host iron formations of the Asbesheuwels Iron Formation. These units dip outwards at less than 10 degrees on the eastern margin of the dome. In the central and western sections of the dome, this part of the sequence is concealed by the unconformably overlying red-bed clastic Gamagara Formation of the Olifantshoek Group.

A unit of ferruginous chert breccia (the Wolhaarkop Breccia) which grades upwards into a distorted banded iron formation (the Manganore Iron Formation) is wedged between the underlying Campbellrand carbonates and the unconformity at the base of the Gamagara Formation. The Wolhaarkop Breccia is matrix supported and consists of unsorted angular chert fragments in a hematite to manganese bearing siliceous matrix.

Some 80 to 90% of the ore at the north-south elongated 12 x 1.5 km Sishen deposit is hosted by the Manganore Iron Formation, which is correlated with the Asbesheuwels Iron Formation, and is found immediately below the unconformity with the overlying clastic Gamagara Formation. Subsequent to deposition, Asbesheuwels Iron Formations are interpreted to have locally slumped onto a palaeo-sinkhole dominated surface developed in the underlying Cambellrand sub-group carbonates to produce the Wolhaarkop Breccia and Manganore Iron Formation during the period of erosion prior to the deposition of the Gamagara Formation.

Silica is believed to have been leached from the slumped and brecciated iron formation by alkaline supergene fluids at the time of slumping, while ferrous ions were oxidised to hematite, and additional transported supergene iron was added. Erosion of the hematite mineralisation and resultant accumulations of hematite pebble conglomerates in alluvial fan environments are preserved as the Doornfontein Conglomerate at the base of the Gamagara formation and account for around 10-20% of the orebody. The Doornfontein Conglomerate is best developed immediately adjacent to pockets of Manganore Iron Formation.

The Manganore Iron Formation is more restricted in areal extent than the Wolhaarkop Breccia. It is composed of 7 zones/bands from the base, namely: Zone 1 - a spotted carbonaceous and dark brown shale with chert pillows and hematite nodules; Zone 2 - hematite micro-banded white chert with interbeds of intercalated cherts and black-brown shale; Zone 3 - chert banded hematite ferhythmites with cycles of hematite-lutite to hematite-microbanded chert, to hematite ribbon, wave and pillow-rhythmite; Zone 4 - hematite rhythmites that represent the bulk of the banded ore; Zones 5 and 6 of hematite-greenalite banded lutites. The first 6 zones were derived from the Kuruman Iron Formation, the lower of the 2 Asbesheuwels units, while a zone 7 composed of hematite lutite with meso-bands of peloidlutite is correlated with the overlying Griquatown Iron Formation. Chert bands within the Manganore Iron Formation are porous, with partially infilling platy hematite. Boundaries of high grade ore cut across primary sedimentary boundaries.

The Makganyene diamictite and Ongeluk Lava of the Postmasburg Group which unconformably overlies the Ghaap Group and unconformably underlies the Gamagara Formation of the Olifantshoek Group, were subsequently thrust over the deposit and host sequence from the west. Much of the sequence in the region is concealed by around 50 m of Tertiary Kalahari Formation cover.

The 2001 mineral reserve estimate for the Sishen deposit was 877 Mt, and the mineral resource estimate was 1724 Mt. Some 70 km to the south at the Sishen South (or Welgevonden) deposit there is a high quality resource of 259 Mt of high lump ore to be developed by 2006.

The 10 mtpa Khumani iron ore mine (Assmang Limited), formerly the Bruce, King and Mokaning ("BKM") Project which refers to the farms on which the iron ore resources are located, is situated in the Northern Cape province of South Africa. The iron ore deposits are located adjacent to Kumba Iron Ore's Sishen mine, and approximately 60 km north of Assmang's Beeshoek mine.

As at Sishen, the deposits are hosted within a sequence of Palaeoproterozoic sediments of the Transvaal Supergroup and lie on a limb of the Maremane anticline. The ore is situated along the contact between the Gamagara formation and the underlying Manganore iron formation in the northern part of the regional domal structure. Two ore types are present: i). laminated hematitic ore of the Manganore iron formation, similar to that at Sishen, and ii). conglomerate ore of the Doornfontein Conglomerate member at the base of the Gamagara formation. The distribution of the Manganore iron formation is restricted, only being preserved in pockets above the Wolhaarkop Breccia below the Gamagara unconformity. The basal Doornfontein Conglomerate is best developed where it overlies the Manganore iron formation. The Khumani deposits are characterised by large stratiform bodies and prominent hanging wall outcrops. The down-dip portions of the iron formation are well developed, although in outcrop, the deposits are thin and isolated.

Hematite is the dominant ore mineral, although limonite and specularite are also present. The K2O content of the ore apprears to be directly related to the abundance of muscovite and illite, while the Al2O3 is the result of kaolinite and muscovite in the ore. The ore is depleted in Na and P, with the latter occurring as trace apatite.

Ore is present in two main areas, at Bruce and King, which are approximately 10 km apart. The Bruce orebodies are to be mined from three main and four or five satellite pits, some of which abut the eastern Sishen boundary. The larger Bruce pits will cover areas of from 2000 to 750 m x 750 to 500 m in area with final depths of 95 to 170 m. The King orebody will be mined as part of the expansion to a 16 mtpa rate in two pits, the larger of which will cover an area of 3000x1000 m, exploiting near surface ore to the west, and following it down dip to ultimate depths of 230 m.

Iron ore is mined from a series of open pits and hauled to the primary and secondary crushers, from where, it is transferred by conveyor to stockpiles ahead of the beneficiation plant. The run-of-mine ore will be stockpiled on blending beds in two categories, 'on grade' and 'off grade'. On-grade material will then be washed and screened to produce the final products incorporating tertiary crushing of the oversize material from the screening plant. Off-grade material will also be washed and screened and the oversized material crushed in the tertiary crushers and, thereafter, beneficiated through jigs to remove contaminants. Products will be stockpiled ahead of transport via the 860 km Sishen-Saldanha railway line to the port of Saldanha Bay on South Africa's West Coast from which the iron ore is being exported. Three products are being produced, lumpy, DR lump and fines, with respective grades of 66, 65.5 and 65% Fe.

A one day workshop will be led by internationally renowned expert Dr Carlos Alberto Rosiére of the Universidade Federal de Minas Gerais in Belo Horizonte providing an outline of the distribution, setting, geology and mineralisation of the major iron producing districts and deposits of Brazil.

Brazil is the world's largest iron ore exporter with an annual production of over 250 Mt, of which over 70% is exported. The majority of this comes from two regions, the Serra dos Carajás in Para State in the north, and the Quadrilatero Ferrifero further south in Minas Gerais State. The Carajás ores are found within Archaean iron formations, while those in Minas Gerais are hosted by Palaeoproterozoic BIFs. In addition however, high grade, high lump ore is extracted from the Corumba - Urucum district in the state of Mato Grosso do Sul to the west from within Neoproterozoic hosts.

The summaries above were prepared by T M (Mike) Porter from a wide range of sources, both published and un-published. Most of these sources are listed on the "Tour Literature Collection" available from the Iron 2010 Tour options page.